1. Field of the Invention
The present invention relates to neutralizing agents for effective but mild deacidification of books and also other printed and paper products of all kinds, such as newspapers or files for example.
The aging which occurs with all printed and paper products, especially books, during storage leads in particular to progressive damage to the paper material from traces of acids released in the paper. If no counter-measures are taken in the way of neutralizing these acids, the damage leads to complete destruction of the paper after some decades. In order to keep archive and library stocks of worldwide some hundreds of millions of books and other archive material it is therefore necessary to neutralize the acids in the paper and at the same time to bind to the paper a sufficient amount of a substance which will act as an alkali reserve and neutralize acids still to be released in the paper in the future. Having regard to the very large numbers of books, the only methods which are suitable are those which allow the treatment of whole books, i.e. methods in which it is not necessary to open the book binding and treat the pages individually. The same applies to archive materials of all kinds, which likewise require treatment of sheets of paper which are bound up or otherwise held together. The need to treat bound archives gives rise to the very important requirement that the neutralizing agent and in particular the solvent required for the treatment shall in no wise attack or modify the components of the archives, such as glues and adhesives, binding materials, printing and stamping inks as well as writing inks. Since the archives are to be treated in bound form, it is unavoidable that the individual sheets lie more or less tightly against each other during the treatment. For this reason it is especially important that printing inks, stamping inks and writing inks, with which the books are signed in many cases shall not be dissolved in any wise, so that bleeding of the writing on to the adjacent sheet will be avoided.
2. Description of the Prior Art
Of the known methods of de-acidification special importance attaches to those methods which employ compounds of the alkaline earth metals for the neutralization. Alkaline earth metals form very stable salts with the acids in the paper, which ensure reliable conservation and provide an alkaline reserve in the paper with pH values of around 8-9, which are compatible with the paper and do not cause any alkaline damage.
One known method for de-acidification of archive materials (U.S. Pat. No. 3,969,549) consists in treatment of the books with vapors of alkyl metals, especially with vapors of diethyl-zinc. Through the moisture in the paper, the alkyl metal is converted into the oxide of the metal, e.g. zinc oxide, which remains in the paper and forms a good neutralizing agent for free acids. The alkyl metals suitable for this purpose are however materials which decompose spontaneously in air, which present a constant risk of fire and explosion in their handling and therefore call for an extreme degree of care and corresponding qualifications.
According to another known method (EP 0 273 903 A2) the books are impregnated with an aqueous solution of calcium hydroxide, whereby free acids are bound as calcium salts. The treated and still wet books are shock frozen and freeze dried. Although effective de-acidification is possible with this method, it has to be recognized that, as is generally known, water acts unfavorably on paper, especially books. As well as curling of the paper and especially of book covers there is a risk of damaging the book glue and of washing out the inks and stamp inks. Furthermore, there is a risk of the ice crystals damaging the paper in the freeze drying, in spite of the shock freezing. Very long drying times are associated with the freeze drying, which can amount to 34 hours and make a sensible pattern of work impossible.
A further known method, the so-called Wei T'O method, which has been used in conservation practice since 1974, uses a solution of a magnesium organic compound for the treatment, such as methyl magnesium carbonate for example. This method is referred to also in the article "Mass de-acidification in France" by Jean-Marie Arnoult, Restaurator 22/13 984, 1987. In this method also the magnesium compounds are converted into magnesium oxide and magnesium carbonate by the moisture in the paper, these both being adapted to neutralize acids. Methyl magnesium carbonate is however like all alcoholates of alkali and alkaline earth metals insoluble in non-polar solvents. However, non-polar solvents are in fact especially suitable solvents because of their compatibility with the archive materials. The Wei T'O method therefore uses as solvents which are compatible with books such solvents as fluorochloro-hydrocarbons for example, the solubility of the methyl magnesium carbonate being achieved by the addition of 10 to 20% of low boiling alcohols, such as methanol or ethanol. In practice it is found that these alcohol additives are enough to effect leaching out and bleeding on to the adjacent sheets of inks and stamping inks.
The cited problems in relation to the solvent are avoided in another known method (U.S. Ser. No. 252,421) in which magnesium glycolates are used. These glysolates also dissolve in non-polar solvents, such as trichlor-trifluor-ethane or hexane. However with the use of magnesium alcoholates of lower alcohols such as methanols, ethanols, butanols or even propanols, these alcohols are formed in the reaction with the moisture in the paper, as well as magnesium oxide, and vaporize in the ensuing drying on account of their volatility, with the use of glycolates, glycols result and remain in the paper on account of their higher boiling point. If not enough magnesium oxide is put into the paper to affect neutralization and to form an additional alkaline reserve in the paper, a corresponding amount of glycol unavoidably results, which remains in the paper and lends this a moist to greasy character.